In the petroleum industry, many producing zones in wells comprise an unconsolidated sand or sandstone type formation having petroleum within. Were an operator to permit the petroleum to flow from the producing zone into the open end of a production string, or into large apertures in the production string, the formation would break down and clog the production string; moreover, the formation could collapse in the vicinity of the well bore "killing" the well by greatly reducing its own permeability and thus damaging it for production purposes due to reduced or terminated petroleum flow.
One way of stabilizing a sand or sandstone type producing zone is to place a "gravel pack" between the formation and the production string. The gravel pack presents a barrier to migrating sand from the formation while permitting fluid flow. To effect a gravel pack in a "single zone" or single producing formation well, a sand screen is suspended at the bottom of a liner from a liner hanger, which in turn is anchored to surrounding well bore casing. If the well is unlined, the sand screen is incorporated in the casing. A sand screen comprises, in one common embodiment, a length of pipe having apertures through the wall thereof, with wires wound around the apertured portion of the pipe in such a fashion as to create only very small intervals between adjacent strands of wire. The wire is generally welded to ribs on the exterior of the apertured portion of the pipe. The bottom of the pipe is closed. The sand screen is of a length greater than the width of the producing zone, and is placed adjacent thereto in the well bore so as to extend above and below the formation.
In the gravel packing operation, drilling mud and other contaminants are usually washed from the well bore, and the formation treated. Common treatments include acidizing to dissolve formation clays, and injecting stabilizing gels to prevent migration of formation components and formation breakdown prior to packing.
In gravel packing a single producing zone, a packer is set above the producing zone between the liner and the casing or between casing and well bore wall, if no liner is employed. A tubing string is run inside a liner assembly (or casing) to a level just above the zone, and a gravel slurry is pumped down the tubing/liner (or casing) annulus out into the annulus between the liner and casing or casing and well bore wall. The "gravel," which in many instances is merely a very coarse grade of sand itself, is sized so as to be larger than the distance between the wires of the sand screen, thereby being deposited on the outside of the screen and settling into a "pack" as the slurry carrier fluid enters the screen and is returned to the surface, generally by using a crossover tool.
After a period of time, the gravel pack builds up around the sand screen until it reaches a level higher than that of the highest screen apertures. At that point, pumping pressures at the surface become noticeably higher, and the slurry pumping operation is stopped. If desired, the return tubing string can then be shut (or the crossover closed if one is employed) and pressure applied in the same direction as the slurry flow, to squeeze the pack into the formation to consolidate the pack. After squeezing, the crossover tool is opened and a clean fluid is "reverse circulated" by pumping down the tubing string to the level of the gravel pack inside the sane screen and back up to the surface to flush out the interior of the sand screen. Subsequently, the well may be subjected to other treatments if necessary, and produced.
Once the well has been gravel packed, however, the operator must choose between using the tubing string with crossover tool in place for production, and removing it and inserting a simple production string. If the latter approach is taken, there is no downhole closure of the producing zone while the tubing is out of the well. Prior art shut in devices are known, but these prohibit entry of a wash pipe down into the sand screen area, a desirable feature during reverse circulation. Furthermore, prior art shut in devices do not provide for automatic closure when the tubing string is removed. Finally, prior art devices are not susceptible to operation by a production string inserted after the tubing string is removed, as the shut in devices of the prior art are connected to the tubing string.